Voltammetry is a group of electroanatytical methods where one achieves a current signal due to a redox reaction at a given potential. The current is controlled by the mass which diffuses to the electrode surface and there is a linear relation between signal and concentration of the analyte.
Various voltammetric methods are important in the determination of compounds in trace levels, with the additional advantage that it can be used not only to determine the total content, but this can also be used for speciation studies in the environment [1-3].
In general, however, direct voltammetric methods are not always sufficiently sensitive to allow trace determinations of pollutants like heavy metals and pesticides. However, the use of different pulse techniques are well known procedures to increase the sensitivity, but even higher sensitivity is required for trace determinations in natural surroundings. One way to increase the sensitivity is to preconcentrate the actual sample. Using various stripping techniques can perform this, and such methods will not normally contaminate the sample. Another way of increasing the signal is to modulate the voltammetric curve with alternating current or potential pulses, or by combining the most appropriate of the above mentioned techniques, like in differential pulse stripping voltammetry, with satisfactory results. However, one problem has to be considered. The increase in sensitivity is only interesting if the noise level is not increased accordingly.
Differential pulse anodic stripping voltammetry (DPASV) is an electrochemical method of analysis where one achieves especially good sensitivity as a result of a combination of a preconcentration step and an advanced measuring procedure which gives a favourable signal-to-noise ratio. The preconcentration is performed by holding the working electrode at a negative potential, then reducing metal ions from the solution to amalgam on a mercury electrode. The metal is preconcentrated into the electrode by a factor of 100 to 1000, and one may determine four to six metals simultaneously in mixtures with concentrations down to 10−10 M.
During the preconcentration step the solution is stirred to improve the mass transport to the electrode surface. The mechanical stirring has two major effects: 1) mechanical transport of ions to the electrode, 2) reducing the thickness of the diffusion layer. The diffusion layer is the area close to the electrode where the concentration of the actual species is lower than the concentration in the bulk of the solution, due to the electrode process. Diffusion is a slow and controlling process and one therefore wishes to lower the diffusion layer to achieve an increased mass transport to the electrode surface where the redox reaction is taking place.
Stirring of a solution in connection with voltammetric measurements, like in DPASV, will give an increase in signal. This is due to the fact that mass transport to the electrode will be facilitated, corresponding to an effective decrease of the thickness of the diffusion layer. But on the other hand, stirring (e.g. with a magnetic stirrer) will be unfavourable for the reproducibility thus being carried out during the preconcentration step only, and normally skipped during the registration of the signal.
The concentration gradient formed close to the surface of the electrode lowers the surface concentration of the actual species. The use of a magnetic stirrer is a well-known technique, by diffusion layer thinning, to increase the surface concentration.
Several techniques have been introduced recently in order to improve the sensitivity and reproducibility of various voltammetric methods of analyses. Among such techniques, rotating electrodes are frequently used [4-7]. Here the rotation has the similar effect an sensitivity as stirring, but being much more reproducible. Some voltammetric experiments with rotating electrodes are reported using stirring during the entire measurement cycles, but normally such investigations are carried out with unstirred or unrotated conditions during the scanning step in order to achieve optimum reproducibility [8, 9]. Other techniques [10-21] have also been introduced to increase the sensitivity and applicability of voltammetric analyses. Of great importance is the use of chemical modified electrodes [22-24]. The application of microelectrodes and ultramicroelectrodes is also an important field in order to improve the detection limit and to allow new applications of the analytical method [25, 26]. Other recent developments are the flow sensor electrode [27-29] and the introduction of the band array microelectrode [30).
The application of ultrasound in voltammetry is a well-established technique [31-39], and a field in fast development. The favourable effects by using ultrasound is mainly due to the following: Increased mass transport of the redox species to the surface of the electrode, an efficient and continuous cleaning of the electrode surface, degassing of the electrode surface, and a removal of a possible viscous surface layer on the electrode.
Another reason for using ultrasound in electroanalytical methods is the fact that high-energy ultrasound can activate the actual compounds, and the chemical reactions, through the acoustic cavitation or through acoustic streaming.
There are several physical mechanisms occurring by using ultrasound which can modify the process, e.g. creation of ions, radicals and other intermediate products rich in energy as a consequence of transient cavitation, ultrasonic activation of chemical processes associated with electron transport in steps, continuous cleaning and activation of electrode surfaces, and an increased mass transport as a result of cavitation in the solution. The purpose of using ultrasound in connection with electrochemical analysis are the effects that follow from cavitation.
Cavitation is a phenomenon that occurs for high frequency sound. Small gas bubbles are created in the medium which then collapse and disengages large amounts of energy. There are theories assuming that the temperature locally may reach 5000 K and that the pressure locally may reach a level of several thousands atmospheres for a short time when the bubbles collapse. The bubbles are created and destroyed by the extremely powerful oscillations in the medium. Ultrasound causes the molecules in the medium to stretch far and fast apart beyond the limit of the critical distance that keep the molecules together. The molecules are torn apart and gas bubbles are created. When the medium oscillates back, the bubbles are compressed, and collapse. The high energy and high pressure that follows from a bubble collapse give the special properties to the chemical elements in the medium.
As a summary, in ultrasound exposed analysis the technique is to use the phenomenon of cavitation to reduce the thickness of the diffusion layer, and in this way increasing the mass transport to the electrode surfaces. However, when using ultrasound one has to use a magnetic stirrer for performing the mechanical transport to the diffusion layer.
The fact that rotating electrodes, at least partially, can be kept rotating during the voltammetric scanning, focused on using alternative ways of reproducible mass transport to the electrode.